For immigration advocates rushing to beat back congressional momentum to crack down on so-called “sanctuary cities,” their chief worry isn’t the Republicans who control Capitol Hill.

It’s Sen. Dianne Feinstein.

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The California liberal with a tough-on-crime streak is stirring deep anxiety among immigration activists ever since she said she would write a bill that would force localities to comply with federal immigration requests — prompted by the death of a San Francisco woman, allegedly at the hands of an immigrant here illegally.

Activists, particularly from Feinstein’s home state, have launched protests at her congressional office, demanding that she stop writing legislation they’ve branded as anti-immigrant. More than 50 organizations have written a letter to Feinstein and fellow California Democratic Sen. Barbara Boxer, warning against measures the groups worry would provoke fear within immigrant communities.

And some advocates have even pulled out unflattering comparisons of Feinstein to presidential candidate Donald Trump, who has skyrocketed to the top of national GOP polls with his controversial comments labeling immigrants from Mexico as rapists and drug dealers.

“Feinstein’s bill is trying to get us to the point where we continue to generate fear at the local level,” said Cristina Jimenez, managing director of United We Dream, an immigration advocacy group. “She is basically … joining the Donald Trump bandwagon.”

The furor against Feinstein began to bubble in mid-July, soon after she and Boxer disclosed that they may write legislation in the aftermath of the July 1 shooting death of Kathryn Steinle on a popular San Francisco pier. The suspect in the killing, Juan Francisco Lopez-Sanchez, had been in the United States illegally and had previously been deported to his native Mexico multiple times.

The national outrage over Steinle’s death has roiled the politics of immigration, particularly on the 2016 campaign trail, where Trump and other GOP presidential hopefuls have seized on it to call for tougher immigration enforcement. On Capitol Hill, House Republicans passed a bill last month punishing sanctuary cities, and Steinle’s father, Jim, has delivered two emotional testimonies urging Congress to act.

Despite the uproar, immigration advocates and some Democratic lawmakers are trying to push back against any new federal legislation in response to Steinle’s death, arguing that punitive sanctuary-cities bills could further isolate immigrant communities.

What Feinstein is contemplating is nowhere near as aggressive as the GOP’s proposals. But the advocates’ concern is that the bill, with the stamp of approval from a powerful Democrat, would be a slippery slope and give an enforcement-only strategy on immigration a bipartisan veneer.

And while Boxer has also indicated she may write a bill, much of the anger has been directed toward Feinstein, who’s seen as the main driver behind the legislation considering her slot on the Judiciary Committee and her deep experience on immigration policy.

“What you have is Feinstein giving a knee-jerk reaction … to the demands of people who don’t want immigration reform,” Rep. Luis Gutiérrez (D-Ill.) said.

Immigrant-rights advocates generally oppose policies that call on local law enforcement officials to help carry out federal immigration laws. Doing so, they argue, prevents undocumented immigrants from coming forward to report crimes as witnesses or victims.

But Feinstein — an influential senior senator, as well as a former San Francisco mayor — is becoming a roadblock for activists.

In a brief interview, Feinstein said she wanted to write her legislation as a counterbalance to measures from Republicans, such as a bill from Sens. David Vitter of Louisiana and Jeff Flake of Arizona that would withhold funding for cities and other local jurisdictions that defy federal detainer requests.

A detainer is when immigration officials ask local jails to keep an immigrant in the United States illegally in custody — even if he or she would otherwise be released — which gives federal authorities time to pick up the person. Vitter and Flake’s bill, called the Stop Sanctuary Cities Act, is set for a markup in the Senate Judiciary Committee this week and is a nonstarter with Democrats.

Feinstein said she won’t support the GOP bill and her legislation is meant to be an alternative. As for the backlash against her from advocates, Feinstein said: “I haven’t heard the criticism.”

That’s not for lack of trying on the groups’ part.

Last Thursday, activists from the California Immigrant Youth Justice Alliance staged a sit-in at Feinstein’s San Francisco office, holding signs that read “We are not your scapegoat” while pushing her to drop her legislation. Eight protesters were arrested, the group said.

The letter from immigration advocacy groups and other civil-rights organizations last week described how “deeply concerned” they were that “leaders with the largest immigrant population in the country are willing to consider or lead legislative efforts to mandate local and state police entanglement with federal immigration enforcement.”

And worried immigration advocates have met privately with Feinstein’s aides to air their concerns, although her office is keeping details of the legislation tightly held.

“She, as a Democrat, is apparently trying to take the lead in promoting anti-immigrant legislation and reactive legislation, so we are very disappointed and very concerned,” said Angela Chan, policy director and senior staff attorney for Asian Americans Advancing Justice — Asian Law Caucus.

Feinstein outlined the broad contours of her bill at a Judiciary Committee hearing last month: It would require state and local law enforcement officials to tell Immigration and Customs Enforcement when an immigrant here illegally with a felony conviction is about to be released, if ICE asks for a notification.

Exactly how a felony would be defined is in question; Feinstein may exempt immigration-related felonies — such as illegal reentry by someone who has already been deported — from her bill, according to multiple sources. Also under consideration is withholding some federal cash from localities that don’t comply, according to the sources.

The veteran Democratic senator hews mostly to liberal stances, although she’s shown a more moderate political persona on criminal justice issues.

Feinstein has long touted a tough-on-crime platform, and last year, she adamantly opposed Proposition 47, a measure that would have reclassified some drug and theft crimes from a felony to a misdemeanor — subsequently reducing sentences. California voters approved Proposition 47 in November.

In Steinle’s death, Feinstein criticized the San Francisco Sheriff’s Department for failing to notify federal immigration officials before releasing Lopez-Sanchez and urged the city’s mayor, Ed Lee, to participate in a new immigration enforcement program from the Department of Homeland Security that some immigrant advocates have approached with skepticism.

Feinstein’s push for federal legislation has also been in sharp contrast to the position of California Senate candidate Kamala Harris, the current state attorney general, who has defended sanctuary cities and the discretion of local officials to do what is best for their communities, even if doing so bucks federal immigration orders.

“The concern from the California community is that frankly, on immigration, particularly given that we have the largest immigrant population, [Feinstein] hasn’t really represented immigrant communities and new Americans’ interests,” said Marielena Hincapie, executive director of the National Immigration Law Center.

For advocates, it’s not just that Feinstein is working on legislation enhancing policies they oppose. They also fear her enforcement-centric bill may peel off support from other Senate Democrats — threatening to splinter a united front that Democrats have had on immigration in recent years.

Most of the immigration enforcement bills unveiled after Steinle’s killing have come from Republicans. Feinstein and Boxer are the only Democrats who have actively mulled legislation, although some Senate Democrats have said they are open to at least considering the measure.

In the House, all but a half-dozen Democrats opposed the bill targeting sanctuary cities that passed last month. Some House Democrats, including California Reps. Nancy Pelosi — the House minority leader — and Zoe Lofgren, have suggested that better communication among the federal agencies involved in Lopez-Sanchez’s case could have prevented him from being released directly to San Francisco officials.

“In contrast to the discipline and party unity we saw in the House, the concern is that what Feinstein will do in the Senate will be divisive,” said Chris Newman, legal director for the National Day Laborers Organizing Network, an advocacy group based in Los Angeles. “It’s the Trump effect. It’s her rush to judgment. It’s her willingness to scapegoat.”

The criticism against Feinstein comes even though she has generally supported pro-immigration policies in Congress.

Like every other Democrat, Feinstein voted for the Senate’s comprehensive immigration legislation in 2013, and she played a pivotal role in the original bill by brokering a deal on agricultural workers. She was also among the dozen senators from both parties who negotiated an immigration compromise in 2007 — when a coalition of labor-aligned liberals, moderate Democrats and conservative Republicans banded together to kill the bill.

Despite the anger exploding from some immigration groups, other influential advocacy groups are reserving judgment on Feinstein’s measure.

“We want to work with her to ensure that if anything is done, it’s done in a way that doesn’t blame wholesome immigrant communities for what happened, but upholds public safety,” said Kevin Appleby, director of migration policy for the U.S. Conference of Catholic Bishops.

Clarissa Martínez-De-Castro, deputy vice president at the National Council of La Raza, said her group would have to see the fine print of Feinstein’s measure before weighing in, but she urged Congress to “legislate responsibly rather than impulsively.”

“Attempts to criminalize a community wholesale are not only misguided,” she said. “They make law enforcement’s job actually harder.”